Exhaust after-treatment systems are needed for many diesel engines to comply with emissions requirements. After-treatment systems such as those needed for Tier 4b and Tier 4f emission regulations include various subsystems and components. The subsystems and components may include particulate reduction systems such as DOC (diesel oxidation catalyst) units and DPF (diesel particulate filter) units as well as NOx (nitrogen oxides) reduction systems such as SCR (selective catalytic reduction) units. SCR units reduce emission of NOx emissions by using ammonia to convert NOx into nitrogen and water within the SCR unit. Since concentrated ammonia can be caustic and hazardous, it is not stored in ammonia form or directly introduced as ammonia into the SCR units. Instead, urea-based DEF (diesel exhaust fluid) is introduced into or near the SCR units and its urea is converted into the ammonia that reacts with the NOx, industry standards require DEF to include about 32.5% urea and about 67.5% deionized water, which can freeze at about 12° F. A high temperature is required to convert the urea to ammonia by vaporizing the water in the DEF and thermally decomposing the urea to form the ammonia that reacts with the NOx to convert it to nitrogen and water, in order to accommodate the needed high temperatures while avoiding freezing problems in cold environments, some DEF systems include dosing systems with doser modules that inject DEF into hot exhaust airflows in the exhaust system, for example, directly upstream of the SCR units and have purging systems that remove DEF from delivery system components during purge cycles and return it to a DEF storage tank to reduce the likelihood of breakage of the components by the expansion of freezing DEF. Components of the delivery system include a supply module that receives DEF from the DEF storage tank and delivers DEF to a doser module. The doser module is an electronically operated module that injects the DEF into the exhaust system. If the doser module injects DEF at too low of an exhaust temperature, not only will the urea fail to convert to ammonia, but the DEF can crystallize on the doser module injection ports causing the doser module to plug. When the doser module plugs, the DEF system purge cycle can allow DEF to be pulled back into the supply module instead of leaving it empty. That is because, when the doser module is plugged, a suction applied to the supply module in order to remove DEF from it and the doser module can establish a siphon in the supply module feed line, which draws additional DEF from the DEF storage tank and reintroduces it into the supply module. In cold environments, the DEF that was inadvertently reintroduced into the supply module by siphoning during the purge cycle can freeze and expand, which can break the supply module.